1. Field of the Invention
The present invention relates to a camera with an autofocus function, and an autofocus control method.
2. Description of the Related Art
In general, a digital camera is provided with an autofocus (AF) function. For example, in accordance with a photographer's half-push operation on the shutter button, focusing on a subject can automatically be made. An example of the method of the AF function is a contract method in which autofocus is made by making use of pixel data that is acquired from an image capturing device (e.g. charge-coupled device (CCD)).
In the AF process by the contrast method, a contrast of an image is detected on the basis of pixel data that is obtained by imaging a subject. By moving the focusing lens to a lens position where the contrast value takes a maximum value, focusing on the subject can be effected. This contrast method is characterized in that when the image of the subject is in a blurring state, the contrast of the image is low, and the contrast value gradually increases as the image of the image becomes closer to the in-focus state, and the contrast value takes a maximum value when the image is in the in-focus state.
In the AF process, the following control is executed. That is, while the focus lens is being driven with a predetermined pitch, the contrast value is detected on the basis of pixel data that is acquired at each of lens positions. The lens position, at which the contrast takes a peak value, is determined to be the in-focus position of the focusing lens, and the focusing lens is moved to the in-focus position.
In general, AF processes using the contrast method fall into two categories: a method in which only a central part of pixel data obtained by the CCD is extracted and upper and lower parts of the pixel data are cut off, and a method by through-image driving, in which the entire pixel data is normally used.
In the former method, an incomplete image, in which upper and lower parts of the entire pixel data are cut off, is acquired during the AF process. Thus, a through-image (display screen for confirming a subject), which is used as an electronic view-finder image, cannot be displayed. Under the circumstances, during the AF operation, a black screen is being displayed, or a still image, which is acquired prior to the start of the AF operation, is being displayed. In this case, there is such an advantage that the transfer amount of pixel data is reduced since only a part of the entire pixel data is extracted and transferred, and thus the frame rate can be increased and a high-speed AF process is realized.
On the other hand, in the latter method, unlike the former method, there is an advantage that the AF process can be executed while the through-image is being displayed. However, since it is necessary to acquire the entire pixel data, the data transfer amount is large, and the frame rate cannot be increased. The time that is needed for the AF process becomes longer than in the former method.
In the prior art, there has been proposed an electronic camera wherein the speed of the AF control is increased and the preview display at the time of the AF control is made proper (Jpn. Pat. Appln. KOKAI Publication No. 2004-221650). In this prior-art digital camera, the CCD image-capturing device is switched between a draft mode in which a relatively large number of pixels are read out, and an autofocus mode in which pixels can be read out at a relatively high speed. At the time of the AF control, the AF control is executed on the basis of an image that is read out in the autofocus mode, thereby achieving high-speed AF (autofocus). In addition, making use of the image that is read out in the autofocus mode, the amount of movement (vector) of the subject is detected. On the basis of the detected amount of movement and the image that is read out in the draft mode prior to the transition to the autofocus mode, an image for display is generated and the generated image for display is displayed on, for instance, a liquid crystal display (LCD).
In the prior-art digital camera, however, in the case where the high-speed AF process is to be executed, a through-image cannot be displayed. On the other hand, in the case where a through-image is to be displayed during the AF process, the time that is needed for the AF process becomes longer.